Update firewall_restriction.rst

the correction assumes the the custom request matcher is App\Security\CustomRequestMatcher.

the `request_matcher` option is the service id. it should be mentionned before that  the `app.firewall.secured_area.request_matcher` is the service ID / or an alias like the following : 

```
     app.firewall.secured_area.request_matcher:
        alias: App\Security\RequestMatcher
```

Author: issamkhadiri1989

security/firewall_restriction.rst

@@ -215,7 +215,7 @@ If the above options don't fit your needs you can configure any service implemen
         security:
             firewalls:
                 secured_area:
-                    request_matcher: app.firewall.secured_area.request_matcher
+                    request_matcher: App\Security\CustomRequestMatcher
                     # ...
 
     .. code-block:: xml